19 April 2018

FSK two channels in F7B mode, likely an UKR-Net

On 13960.0 KHz I tuned an apparently MFSK-4 100Bd/1000 transmission spreading about 3000Hz bandwidth: tones at -1500, -500, +500, +1500. After emails exchange with friends, "cryptomaster" from radioscanner suggested that the signal is a F7B mode actively used by Ukrainian Nets. Usually, the two channels transfer T-207 ciphered data. He also warned me that SA program parses these transmissions as classic MFSK-4 therefore the received result doesn't correspond
to the truth. Another F7B signal, this one from Ukrainian Mil, was reported here some times ago.

Fig. 1

Fig. 2

F7B
is a FSK modulation technique with four modulation frequencies, the two
transmission channels, termed V1 and V2, are obtained through six
possible combinations of the four frequencies and this leads to 6
different modes F7B1 to F7B6:http://www.wavecom.ch/content/.../twinplex.htm

The equipment BUK-2D, for
example, is one of the equipments used to filter and separate the two channels (thanks to 'cryptomaster').